This invention relates to an irradiated fluoropolymer composition, to heat recoverable articles comprising said composition and to an elongate conductor insulated with said composition.
Thermoplastic polymers of vinylidene fluoride have desirable properties, such as fluid resistance, which make them suitable for various uses such as wire and cable insulation and heat shrinkable tubing. In using thermoplastic vinylidene fluoride polymers in such application, the polymer may be cross-linked, for example by irradiation as described in U.S. Pat. No. 3,580,829 to Lanza, the disclosure of which is incorporated herein by reference. However, thermoplastic vinylidene fluoride polymers in general, have been found to be too stiff and/or too brittle, particularly at temperatures below -20.degree. C., and to have relatively poor elongation.
It has been suggested that vinylidene fluoride polymers, in particular a copolymer of vinylidene fluoride containing about 5% by weight tetrafluoroethylene, be blended with an elastomeric vinylidene fluoride-hexafluoropropylene copolymer to improve the physical properties. See U.S. Pat. No. 3,864,228 to Rossetti. However, as reported in that patent, while the resulting composition may have certain improved properties, such as tensile strength and elongation, the resulting composition has relatively poor heat aging properties. Further, while the elongation is somewhat improved it is still too low for certain uses.
Subsequent work on vinylidene fluoride-tetrafluoroethylene thermoplastic copolymers, as reported in more detail below, supports the conclusion that the addition thereto of an elastomeric vinylidene fluoride-hexafluoropropylene copolymer undesirably and uncontrollably lowers the secant modulus. In certain compositions, depending on the secant modulus of the particular vinylidene fluoride polymer used, the modulus may be too high and elongation too low for use of the composition in preparing flexible heat shrinkable tubing and in others, the modulus may be too low to prepare satisfactory heat shrinkable tubing.
Blends of vinylidene fluoride polymers with the fluoropolymers for other uses are known. See for example U.S. Pat. No. 4,560,737 to Yamamoto et al which relates to piezoelectric materials. Yamamoto et al disclose blends of a vinylidene fluoride base resin, for example vinylidene fluoride homopolymer or copolymers with, for example, tetrafluoroethylene, hexafluoropropylene, or hexafluorobutylene, and a modified fluoroelastomer. The fluoroelastomer is block or graft copolymerized with vinylidene fluoride monomer or a vinylidene fluoride containing monomer mixture. The blend can be used to form a sheet or film having a high piezoelectric modulus.
Our invention provides a cross-linked vinylidene fluoride based polymer composition which possesses the desirable properties of polyvinylidene fluoride yet has a secant modulus and elongation making it suitable for use in preparing flexible heat recoverable articles and as insulation for wire and cable.